trans-Methylpyridine cyclen versus cross-bridged trans-methylpyridine cyclen. Synthesis, acid-base and metal complexation studies (metal = Co2+, Cu2+, and Zn2+).

The synthesis of the cross-bridged cyclen CRpy2 {\{}4,10-bis((pyridin-2-yl)methyl)-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane{\}}, a constrained analog of the previously described trans-dipyridylmethyl cyclen (1,7-dipyridylmethyl-1,4,7,10-tetraazacyclododecane) (Cpy2) is reported. The addnl. ethylene bridge confers to CRpy2 proton-sponge type behavior which was explored by NMR and potentiometric studies. Transition metal complexes were synthesized (by complexation of both ligands with Co2+, Cu2+ and Zn2+) and characterized in soln. and in the solid state. The single crystal x-ray structures of [CoCpy2]2+, [CuCpy2]2+ and [ZnCpy2]2+ complexes were detd. Stability consts. of the complexes, including those of the cross-bridged deriv., were detd. using potentiometric titrn. data and the kinetic inertness of the [CuCRpy2]2+ complex in an acidic medium (half-life values) was evaluated by spectrophotometry. The pre-organized structure of the cross-bridged ligand imposes an addnl. strain for the complexation leading to complexes with smaller thermodn. stability in comparison with the related nonbridged ligand. The electrochem. study involving cyclic voltammetry underlines the importance of the ethylene cross-bridge on the redox properties of the transition metal complexes. [on SciFinder(R)]


Soumis le 12 avril 2018